Hey guys, one of the questions I get all the time is based on the lockout temperature on heat pumps and I'll have uh, more homeowners than contractors I Think most contractors understand what's going on, but a lot of homeowners seem to be misinformed on what this is all about and why certain systems lock out the heat pump depending on what the backup heat source is. So what I'm talking about is say if you have a dual fuel system and you have the thermostat or the dual fuel kit or however, it's set up. dual fuel kits are kind of a thing in the past, but some systems still have them. But if it's set up to say lockout at 35 degrees and then switch to the gas furnace, a lot of homeowners seem to think that that is because of efficiency.

They think that if the heat pump is locking out, it's only because the gas furnace is going to be more efficient than the heat pump, and that's why it's locking it out. Now you might argue it is more efficient, right? If the temperatures are so cold that the heat pump is rendered useless or starts to lose capacity, then the gas furnace might be actually more efficient in certain situations. But when we're talking about different types of gases, you know natural gas is still pretty efficient. But you start talking about LP gas and things like that.

It can give it quite pricey to run the furnace. so that's why folks will install dual fuel systems and such. But the reason I think there's so much misinformation with homeowners. or at least some confusion.

And that is because I'll have homeowners asking me what their lockout temperature should be when they have electric backup. And so what? I Started realizing is these folks don't understand why there's even a lockout temperature on a heat pump. So here's the deal. If you have a heat pump with electric backup because the coil is on the return side of the heat strips, the head pressure on the refrigerant will always stay manageable.

Or at least in most situations unless it gets like super warm outside and you're trying to running in heating mode, then maybe not. But in most situations with that coil being on the return side of the electric heat strips, there's no issue with the outdoor unit the heat pump to continue to run even if the electric heat strips are calling. So if auxiliary heat comes on for example, if the there's a two degree discrepancy between the set point temperature and the ambient temperature in the room and the auxiliary heat turns on, there's really no reason for the heat pump to turn off. In that situation, the heat pump is, you know, drawing very few amps in comparison to those heat strips and any little bit of help it can give the better right? And so now in contrast, if we're talking about a dual fuel system, what happens is the coil is on the supply side of the heat exchanger and so the system has to lock out that heat pump system.

Not because of efficiency, not because it's going to save you energy. It's only doing it because of the head pressure on that coil of the refrigerant. The head pressure would get so ridiculously high that it would be locking it out on high pressure anyway, and it would be absorbing so much more wear and tear. So head pressure with I mean if you figure a heat exchanger with a gas furnace, if that temperature rise is say 50 degrees, the head pressure would be ridiculously high when that heat pump is in heating mode.

I Mean you're probably talking. you know, a thousand pounds or more because some systems run over 400. PSI Anyway, in heating mode, when that heat pump is in heating mode, that head pressure on the refrigerant is jumping up over 400 PSI sometimes. So again, just to clear up that confusion, it does not lock out the outdoor unit for efficiency reasons or any other reasons.

It only does it because of the head pressure of the refrigerant. It would just be Sky High You'd be springing leaks left and right because the pressures would be higher than even the design tested pressures that were tested in the factory, for example. So the materials would not be able to withstand those kinds of freshers. and that's why it needs to lock out that heat pump when the gas furnace turns on anyway.

I Feel like I'm might have got a little more technical than I usually do in a lot of my videos, but I wanted to explain that to you. You know if you're a homeowner and you're trying to figure out, you know, setting up your dual fuel system, setting up the programming, or in contrast, setting up a straight all-electric heat pump system, Why there are lockouts, and why that system needs to turn off the outdoor unit before it can turn on the gas furnace anyway. I Hope that helps. Thanks for watching.

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